I just faced the same problem about a week ago. I had a concrete patio that had been painted before. Trick is to figure out exactly what is on the surface to begin with. Is it epoxy, urethane, or stain? I'm assuming since you say sealed it's coated or stained with something.
Anyway, what I ended up using was Armor Seal 1K water based urethane from SW to coat the patio with. I first sanded the old finish with 50 grit paper to give the urethane some good tooth, then swept clean and tacked with a damp cloth. After allowing any left over moisture to dry, I applied two coats of the urethane and it came out beautiful. The finish is hard and smooth with a nice sheen. Depending on your surface (and in my case a small patio area and the extra from a whole gallon) you really only need one good coat. Smallest size being a gallon, I had three times as much as needed, so I threw an extra coat on for good measure.
You could also look into a two part epoxy, but I wouldn't try that if you're unsure of what type of coating is on the surface to begin with. The epoxy could react and cause poor adhesion if mixed with a different type of previous coating.
Hope that helps! I'm by no means a concrete expert, but I'll give what I can! :Thumbs:
Pro, I'm not too sure about acid in a basement, I've had the fumes attack metal in a garage with big fans going. Sanding doesn't sound like much fun either and we all know about concrete dust.
I use a primer called Skip Sand or Bondcote. It is a marine primer used to bond bottom paint to new fiberglass boats without sanding. Ever manufacturer has their own name for the stuff. It may cost a little more but sure beats the mess that the other two options create.
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